1. Field
Aspects herein generally relate to rheometer systems for testing polymers, and more particularly to a decoupled cross-head incorporated into a rheometer system.
2. Discussion of Related Art
Polymers are often tested according to one of several ASTM methods, namely, ASTM D1646, D2084, D5289, and D6204. Instruments operating in accordance with ASTM D2084 and D5289 are known. For example, U.S. Pat. No. 3,681,980 illustrates the application of a fixed eccentric cam to facilitate oscillation of a rotor. This amplitude of oscillation is determined by the position of the pin on the eccentric. U.S. Pat. No. 4,794,788 also illustrates the use of an eccentric to facilitate an oscillatory motion. The amplitude of oscillation can be changed between tests by changing the position of the pin on the eccentric or by changing the eccentric to one with a different off-set.
ASTM D6204 describes the use of a variable frequency test, and also discloses the capability of performing a variable temperature test. ASTM D6601 describes the conditions for evaluating a specimen at more than one strain amplitude during a single test. This test may be used with the apparatus described in U.S. Pat. Nos. 4,794,788, 5,079,956 or 6,681,617.
Many of the apparatus described in these patents and used in the foregoing ASTM test methods are referred to as moving die rheometers. In typical moving die rheometers, two opposing co-axial dies compress a test specimen between them. One die is driven in an oscillatory manner, while the opposite die is free to rotate independently of the first die. A flex arm is connected to the one die, and this flex arm is driven back and forth to create the oscillatory movement of the one die. In these existing systems, a drive system may comprise an eccentric attached to the output of a motor. The eccentric is connected to a link arm which is further connected to a flex arm. The amplitude of movement of the one die is determined by the distance between the axis of rotation of the eccentric and the post of the eccentric.
In other rheometer systems, the drive shaft of the motor may be directly coupled to the one die without the use of any link arms. The desired oscillatory motion is produced by the motor.
Both types of rheometers may employ a moving cross-head driven by an air cylinder to urge the opposite die toward the one die to compress a test specimen between the two dies. In existing systems, the cylinder shaft is rigidly coupled to the cross-head. In such systems, the system frequently stutters or binds up, causing problems. Also, alignment of the two opposing coaxial dies is sometimes difficult to maintain.